When a scanning light beam, such as a laser beam, is used to create an image on a media, such as a photographic paper, film, or other photosensitive media, it is of utmost importance to locate the starting point for each scan line as precisely as possible. If successive scan line starts differ by too great an amount, the vertical details of the image would be lost. In other words, a vertical line in the image would appear wavy or jagged. The amount of precision required for this type system was established empirically at .+-.10 nSec. Current detection systems have a precision of .+-.50 nSec. In prior art devices, a photodiode is typically used for detecting the beam and for calculating the light start of the scanning light beam. A problem associated with typical circuits that are used for controlling the photodiode is that some dark current is always present. With high gain transimpedance amplifiers, this can lead to large amounts of dc offset in the output, which can cause the detector to be triggered at the wrong time. This dc offset is normally compensated for by including an offset nulling adjustment to one of the inputs of the transimpedance amplifier. However, this is a manual and very tedious adjustment. Additionally, it is subject to exactly how much ambient light is hitting the detector, how much patience the operator has, and how much gain is in the transimpedance amplifier.
The present invention provides a circuit whereby the problems of the prior art are minimized and/or eliminated.